This invention relates to head up display systems, and in particular to high accuracy head up display systems in which an image may be superimposed exactly over an outside view such as a weapon aiming marker for an aircraft.
A conventional head up display (HUD) system includes a light emissive object (usually a cathode ray tube (CRT)) situated at an object plane and optics, including a collimator and a combiner for providing a collimated image of the object at an image plane for viewing at an eye plane combined with an outside view through the combiner. A feature of HUD systems design is the close attention which must be paid to aberrations within the optics to ensure that an accurate and correctly collimated image of the object is provided over the full field of view. This is particularly important in systems where images are required to be accurately placed with respect to the outside world, such as aircraft weapon aiming or way point markings, in addition to data presentation.
Use of head up displays has proved so important in military aviation, that the accuracy and quality of the display are now regarded as among the limiting factors of operational performance. This importance has led to a desire to present an increasing amount of information by means of HUD, and display clutter due to the large amount of information displayed is now a major problem in the art. In order to provide a solution to this problem the use of color attention cues has been suggested.
Existing head up display systems concentrate on providing an accurate image from a monochrome source, so objects producing color displays would give rise to collimation and positional errors with observer head movement. Full color correction would involve increased weight and cost and is not considered practical and hence all known electronic head up display systems presently installed in aircraft are monochrome only in that the optics are optimally accurate at a single wavelength.
If an attempt is made to use a monochrome only system with a colour object because the system does not have full chromatic correction the system focal length will change by a small amount for light of wavelengths away from the wavelength for optimum accuracy. Hence the system will have inadequate accuracy and an incorrect image plane which will be evident with observer head movement. Collimation errors which result in movement of the image relative to the outside view with head movement clearly rule out any display of ground locked images such as sights and target markers. Such errors are particularly damaging if present in high performance systems with a large exit pupil, in which a design aim is to tolerance of head movement within the exit pupil volume.